SMAs are used in small actuators because of their ability to change shape by heating, and their low weight and high force/weight ratio. Several mini actuators have been proposed using SMAs.
When the size and the forces generated required by the design increase, SMAs are used in combination with a frame or body. The expansion and contraction of the SMA component(s) drives the motion of one frame or body part relative to a base part. Most such devices use SMA wires in conjunction with a spring or springs to restore the position of the actuator after motion caused by the change in shape of the SMA components.
As an alternative to providing the restoring force by means of a spring, the use of SMA components acting antagonistically has been proposed. Pairs of SMA components such as wires or springs are arranged to provide motion in contrary directions when powered. This provides positive drive in each direction of motion, rather than relying on a restoring force for one direction.
In some proposed devices tension springs are used in an in line configuration to moderate the forces provided by the SMA components to the driven parts. For example in Smart Materials and Structures 22 (2013) 085006, Lai et al propose a miniature manipulator with two degrees of freedom provided by rotation of two links about two perpendicular axes. The rotations are each driven by pairs of antagonistic SMA wires mounted from a fixed base to the rotating part. Tension springs are fitted in series with each SMA wire, intervening between the wire and the moving part, to reduce stress on the SMA components.
When designing an actuator making use of SMA components for manipulating the direction of at least part of an endoscope, or to provide motion to a snake-like robot (SNLR), a number of issues arise. The heat generated in use of the SMA components in use must be dissipated and/or kept to an acceptable level. Other issues include the relatively small extent of motion provides by SMA wires, typically no more than 5% contraction on heat activation; and the desire for compact arrangements that can nevertheless provide complex motion.